l(U) A=a, Z=z=0   (a,z) A Z       l(u) = 0    U-a.e.


Figure REF  shows the causal graph describing these relationships. In our setting, we assume that there is no observable confounder, though this may be easily included [1]}, [2]} as presented in Appendix .
Here, the bidirectional arrows mean that we allow both directions or even a common ancestor variable.
Given these assumptions, it is shown that the structural function can be expressed using a solution of an integral equation.
